A number of electrophysiological studies have been devoted to the functional organization of trigeminal nucleus oralis (Vo). Only recently have there been anatomical analyses directed toward an understanding of the structural substrates underlying the electrophysiological events. Vo has be implicated in the reception, central processing and modification and the transmission of neural information related to oro-facial nociceptive (pain) and mechanoreceptive (tactile) stimuli and it is perceived as being involved in numerous chronic pain states such as trigeminal neuralgia (tic douloureux). Vo is a major target of primary trigeminal neurons which innervate the tooth pulp where it is generally considered that pain is the only sensation that can be elicited in man. The overall objectives of my studies are to delineate the synaptic circuitry in Vo in order to understand and explain how nociceptive and mechanoreceptive information transmitted from the orofacial region by primary trigeminal neurons is processed before being relayed to other centers in the brain. This project employs light (LM) and electron (EM) microscopy coupled with the methods of anterograde and retrograde transport of horseradish peroxidase (HRP). The specific aims of the research are: 1) to demonstrate the distribution of primary trigeminal afferent axons to the dorsomedial (DM) subdivision of Vo; 2) to examine the morphological features of individual HRP-filled terminal axonal arbors of primary trigeminal afferent axons in DM with LM and then evaluate their cytology and synaptic relations with EM; 3) to compare the overall morphology, cytology and synaptic relations of primary axons terminating in DM with those which have been shown to terminate in the border zone (BZ) and ventrolateral (VL) subdivisions of V0; and 4) to demonstrate the cytology and synaptic connections of identified medullary dorsal horn, cerebellar and spinal projection neurons in DM, BZ, and VL of Vo. The knowledge gained from these studies will continue to provide an essential anatomical framework for understanding the synaptic circuitries through which Vo processes orofacial nociceptive and mechanoreceptive information under normal and pathological circumstances, Such an understanding, in light of concurrent pharmacological advances in drug therapy, should contribute to the successful treatment of trigeminal related chronic pain states which affect the face and oral cavity.